Field of the Invention
The present invention relates to plastic articles such as large diameter plastic pipes used in underground applications. More specifically, the present invention relates to a process and machine for injection molding such pipes.
Description of the Related Art
Plastic pipes for use in underground sewerage and drainage applications are usually manufactured by means of an extrusion process. The wall thickness of these pipes is considerable, in view of the crushing strength required of the pipes. To reduce costs, pipes of lesser wall thickness that are corrugated or ribbed are commonly used for such applications.
Prior art extrusion processes required several separate molds to form a large diameter ribbed length of pipe. Many of these machines have ten or more mold sets. These molds are shuttled from one end of the forming machine to the other in a substantially continuous fashion. The mechanisms used to shuttle the molds and cool these moving molds are often very complicated and very expensive. In addition, the molds themselves are very expensive.
An injection molding process has also been proposed for manufacturing such pipes. U.S. Pat. No. 3,992,503 (the '503 Patent) proposed a method of manufacturing continuous ribbed pipes of thermoplastic resin material without the need to shuttle and cool numerous molds. According to the method proposed by the '503 Patent, a length of pipe is injection molded in a mold cavity between male and female molds, the latter being shaped to provide integral ribbing on the outer surface of the pipe. After sufficient setting, the female mold is disengaged and the pipe length is ejected along the male mold by a plunger, whereupon the female mold is re-engaged with the one end of the formed pipe forming a closure for the mold cavity. Further material is again injected into the cavity which fuses with said one pipe end and which forms a further pipe length. These steps are repeated until the desired length of pipe is formed. It is almost impossible to separate the formed pipe from the male mold by simply pushing the formed pipe with the plunger. Accordingly, heat is applied to melt the inner surface of the pipe next to the male mold so that it can be released and ejected. This is a very ineffective and impractical method.
Accordingly, there is a need for a method and apparatus to injection mold continuous lengths of plastic pipe where the male mold member is separated from the formed pipe prior to the step of ejecting the newly formed pipe from the area of the female mold elements. Alternatively, there is a need for a method and apparatus to injection mold continuous lengths of plastic pipe where the male mold member is used to eject the newly formed pipe from the area of the female mold element and the male mold member is then subsequently separated from the newly formed pipe and returned to its original position.